The meteoritic rise of autonomous navigation in real-world settings for self-driving cars and drones has propelled rapidly growing academic and commercial interest in LIDAR. One of the key application spaces that has yet to be filled, but is of great interest, is a non-mechanically steered LIDAR sensor which has substantial range (e.g., 100-300 m), low power (e.g., 1-10 W), low cost (e.g., hundreds of dollars), high resolution (e.g., 104 to 106 pixels) and small size (e.g., 10 cm3). There are several candidate technologies including micro-mechanical mirrors, liquid-crystal based devices, and integrated photonics that are currently being explored academically and commercially to fill this niche.
Current state-of-the-art chip-scale integrated-photonic LIDARs are based on 1D or 2D phased array antennas. In this type of architecture, a 1D or 2D array of dielectric grating antennas is connected to electrically-controlled thermo-optic (TO) or electro-optic phase shifters. These phase shifters are fed by waveguides splitting off from one main dielectric waveguide which brings power from an off-chip or on-chip source. By applying a gradient to the phases tuning each antenna, in-plane or out-of-plane beam-steering can be enabled.
The direct predecessor of this architecture are radio frequency (RF) phased arrays developed for military and commercial RADARs. Although the detailed implementation is different because RF primarily relies on metallic waveguides and structures whereas integrated photonics uses dielectrics, optical phased arrays are essentially based on directly replacing RF components with their optical equivalents. This direct translation brings a significant disadvantage: whereas metallic waveguides can be spaced at sub-wavelength pitches, eliminating parasitic grating lobes, dielectric waveguides have to be separated by several wavelengths to prevent excessive coupling, resulting in significant grating lobes.
RF phased array radars are routinely produced with closely spaced antennas (<λ/2 apart) in subarrays that can be tiled to create very large apertures. This provides wide-angle steering and scaling to large power-aperture designs. Fundamentally, the radiating elements can be closely spaced with independent control circuitry because the amplifiers, phase shifters and switches in the RF are implemented as subwavelength lumped elements.
Current chip-scale optical phased arrays often reproduce RF phased array architectures, with RF elements replaced with their optical analogs. Fundamentally, the optical analogs to RF components are traveling-wave designs that are multiple wavelengths long and spaced apart by more than λ/2. This design allows beam-steering over very small angles. In an end-fed geometry, for example, the grating antenna elements can be closely spaced for wide-angle azimuthal steering and use wavelength tuning to change the out-coupling angle of the gratings for elevation steering. But this end-fed geometry cannot be tiled without introducing significant grating lobes due to its sparsity.